Several types of surface analysis instruments utilizing x-rays have evolved in recent years. One approach to chemometric surface analysis is electron spectroscopy for chemical analysis (ESCA), also known as x-ray photoelectron spectrometry (XPS), which involves irradiating a sample surface with x-rays and detecting the photoelectrons emitted. The photoelectron energies are characteristic of chemical elements in the surface, and are filtered by an electrostatic or magnetic analyzer which allow only electrons of a specified narrow energy band to pass through to a detector. The intensity of the detected beam typically represents the concentration of a given chemical constituent on or near a specimen surface. U.S. Pat. No. 3,766,381 (Watson) describes such a system that utilizes a hemispherical electrostatic analyzer that transmits electrons of selected energy to a detector. By selecting a range of energies transmitted, analyzing of the electron energies is effected.
A recent development is scanning x-ray photoelectron spectroscopy as disclosed in U.S. Pat. No. 5,315,113 (Larson et al). In this instrument an electron beam is focused on an anode causing emission of x-rays from the irradiated spot. The x-rays are focused by a component such as a concave Bragg crystal monochromator onto the surface of a sample specimen to be chemically analyzed. The monochromator forms an x-ray image of the anode spot on a small pixel area on the surface of the specimen. When the electron beam steps laterally, the x-ray spot on the specimen steps correspondingly from one pixel to the next. The emitted photoelectrons are energy analyzed at each pixel so that a map of the surface chemistry can be constructed. A particular pixel area may be selected for analysis, or the electron beam may be rastered so that the x-ray spot also is rastered for mapping.
The anode for an ESCA instrument typically is magnesium or, particularly in an instrument with a monochromator, aluminum for its K.alpha. x-ray line. Other metals such a tungsten are used for x-ray emissions in other applications. High power is desirable in the electron beam to produce the greatest intensity of x-radiation and thereby attain maximum sensitivity. However, nearly all the power is converted into heat in the anode and can cause melting, diffusion or other deterioration which shortens the useful life of the anode. The normal operating power thus is a compromise between beam intensity and anode lifetime. For this reason, the anode should conduct heat away efficiently. For optimum cooling the anode material is in the form of a thin film of a few microns. In the case of ESCA instruments, substrates for the aluminum or magnesium film typically are water cooled silver or copper, but beam power is still limited. A further problem with a support of silver and copper is erosion of these softer metals by cooling water.
An object of the invention is to provide a novel anode assembly for an x-ray instrument. A particular object is to provide such an anode assembly that is receptive of a focused electron beam to effect x-rays from an anode spot. Another object is to provide such an anode assembly capable of improved life and/or increased x-ray intensity from an anode spot. A further object is to provide an improved apparatus incorporating such an anode assembly for generating x-rays. Yet another object is to provide such an apparatus in the form of an x-ray photoelectron spectrometer for chemical analysis of small areas of a specimen surface.